Any job that you can get your hands on. Future employers just want to see that you are hard working and motivated at a young age. If you already know what you want to do when you grow up you could try finding a job in that field but in general it can be hard to find a job as a 14 year old.
Some ideas are working at your local grocery store, babysitting, doing yard work for people in your community, bussing at a restaurant, or working retail. These are all jobs that require little experience and no specific age requirement.
If you don't have access to many of those types of opportunities and are looking to build your resume, volunteering is a great second option. It boosts your resume and can also be really fun and rewarding. Some volunteering gigs even have steady schedules like a job whereas others can be a one time thing that you sign up for on specific days your available so it can be really flexible if you have other time commitments.